1. Field of the Invention
The present invention relates to a fuel cell including a membrane electrode assembly, and first and second separators sandwiching the membrane electrode assembly. The membrane electrode assembly includes a first electrode, a second electrode, and an electrolyte membrane interposed between the first and second electrodes.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly (MEA) which includes an anode and a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. Generally, a predetermined number of power generation cells are stacked together to form a fuel cell stack.
In the power generation cell, a fuel gas such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen or air (oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Various sealing structures are used for preventing the leakage of the fuel gas and the oxygen-containing gas in the power generation cell. For example, Japanese Laid-Open Patent Publication No. 2002-25587 discloses a fuel cell which is designed to improve sealing characteristics between a membrane electrode assembly and separators.
As shown in FIG. 21, the fuel cell includes a power generation cell formed by a membrane electrode assembly 1 interposed between first and second separators 2a, 2b. The membrane electrode assembly 1 includes an anode 4a, a cathode 4b, and a solid polymer electrolyte membrane 3 interposed between the anode 4a and the cathode 4b. The surface area of the anode 4a is larger than the surface area of the cathode 4b. 
A first seal 5a is attached to an inner surface of a second separator 2b. The first seal 5a is provided around the cathode 4b, and tightly in contact with the solid polymer electrolyte membrane 3. Further, a second seal 5b is provided between the first and second separators 2a, 2b around the first seal 5a. 
In the conventional technique, a space or gap 6 tends to be formed between the cathode 4b and the first seal 5a. In particular, if the second separator 2b is made of metal, at the time of forming the first seal 5a integrally on the second separator 2b using a die (not shown), a die presser surface (planar surface portion) for pressing the die toward the second separator 2b is required. Therefore, the gap 6 corresponding to the die presser surface is formed between the cathode 4b and the first seal 5a. The gap 6 is relatively wide.
It is likely that leakage of the reactant gas through the gap 6 occurs. The reactant gas may flow around the outer portion of the cathode 4b without flowing along a reactant gas flow field (not shown), namely, shortcut of the reactant gas may occur undesirably. Consequently, it is not possible to reliably supply the reactant gas to the reactant gas surface, and thus, the desired power generation performance cannot be maintained.
Likewise, a gap 7 tends to be formed between the first and second seals 5a, 5b. Thus, the reactant gas may flow around the outer portion of the anode 4 undesirably without flowing along a reactant gas flow field (not shown).